Staging: hv: Add the inclusion guard for vstorage.h

In preparation for getting rid of the inclusion of storvsc.c from
blkvsc.c, add inclusion guard to vstorage.h

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>